This invention relates generally to radio frequency circuits and more particularly to radio frequency switch circuits.
As is known in the art, a radio frequency (RF) switch circuit (hereinafter switch or switch circuit) is a device used to connect and disconnect RF signal paths in an electrical circuit. When connecting RF signal paths, a switch generally provides a bi-directional RF signal path. Thus, signals fed to an input port of the switch appear at an output port of the switch and vice-versa.
As is also known in the art, switches may be provided in several configurations including the so-called single pole single-throw (SPST) configuration and the single pole multi-throw (SPMT) configuration.
A single pole single-throw switch includes one input port generally referred to as a common port and one output port generally referred to as a branch port. When the SPST switch provides a signal path having a relatively low insertion loss characteristic between the common port and the branch port, the switch is generally referred to as being in the "on" position. When the switch provides a signal path having a relatively high insertion loss characteristic between the common port and the branch port the switch is generally referred to as being in the "off" position.
A single pole multi-throw switch includes one common port and two or more branch ports. The SPMT switch provides a first signal path having a low insertion loss characteristic between the common port and a selected one of the branch ports. The SPMT switch also provides signal paths having a relatively high insertion loss characteristic between the common port and the unselected branch ports.
Both the SPST and SPMT switches may be provided either in the so-called reflective or non-reflective configurations. When in the off position, a reflective switch reflects signals fed thereto back to the signal source. On the other hand, a non-reflective switch in the off position provides a matched characteristic impedance to signals fed thereto and thus terminates such signals. In the on state, however, both the reflective and non-reflective switch types provide a signal path having a relatively low insertion loss characteristic between the common port and a selected branch port.
The electrical characteristics of an RF switch include isolation, insertion loss, switching speed and RF power handling capability. Depending upon the particular application it is often necessary to optimize one of these electrical characteristics by trading off the performance of the other electrical characteristics of the switch.
In one type of application, RF switch circuits are often used to select between an RF signal source and one of several signal paths or conversely between one of several RF signal sources and a common signal path. For example, radar systems capable of operating at several independent frequencies often include multiple, typically two or three, signal sources. To maintain the frequency stability of such signal sources it is necessary to allow each of the multiple signal sources to continuously provide a signal.
Radar systems, however, generally include a single transmit signal path between the multiple signal sources and a transmit antenna. A switch circuit is used to provide RF connections between each of the signal sources and the transmit signal path of the radar system. Since typical radar systems can, at any instant, only operate at a single frequency it is necessary to provide the radar system transmit path with one signal having a frequency corresponding to that of a selected one of the multiple signal sources. Thus, the switch should provide one RF connection having a low insertion loss characteristic between a selected one of the multiple signal sources and the radar's transmit path. The switch should also provide RF connections having a relatively high insertion loss characteristic between the remaining unselected signal sources and the radar system transmit path.
Since each of the multiple signal sources continuously provide signals, any signals which "leak" between the unselected signal paths of the switch and the selected signal path of the switch will be provided to the transmit path of the radar system. This generally degrades the performance of the radar system. Thus, in this particular application, it is desirable for the switch to provide highly isolated signal paths.
The isolation characteristic of a SPST switch is generally determined by measuring the difference between the power level of a signal provided at the switch output port with the switch on and with the switch off. In the case of a SPMT switch connecting one of several signal sources to a common output signal path, isolation may be defined as the difference in signal power level between the desired signal and undesired signals at the output port of the switch.
RF switches having high isolation characteristics may be provided using PIN diodes. Depending on whether it is desirable to provide a reflective or non-reflective switch, the PIN diodes may be serially coupled along a transmission line or disposed in shunt (i.e. coupled between the transmission line and ground) to provide a switch circuit.
Regardless of whether the PIN diodes are provided in a series or shunt configuration along the transmission line, the desired isolation characteristic of the switch determines the number of diodes required to provide the switch. For example, to provide an isolation characteristic of 80 decibels (dB) typically requires a switch having four PIN diodes with each diode being separated by an electrical path length substantially corresponding to 90 electrical degrees (i.e. a quarter of a wavelength) at the desired frequency of operation.
Several problems exist with the PIN diode approach. First, due to the required diode spacing of 90 electrical degrees, this approach generally provides a relatively large circuit. Second the PIN diode approach generally requires a driver circuit with which to provide bias voltages to switch the PIN diodes. It is often difficult to design and integrate such a driver circuit with the PIN diodes. Third, because of the existence of minority carriers in the intrinsic (I) region of a PIN diode, PIN diode switches are generally limited in their switching speed.
To overcome the size problem and the driver circuit problem of the PIN diode approach, switches have been provided as monolithic integrated circuit field effect transistors (FETs). FET switches typically have a relatively fast switching characteristic however, they have a relatively low isolation characteristic typically of about 50 dB.
Thus, it would be desirable to provide a switch circuit which provides a high isolation characteristic, a relatively fast switching characteristic and which is easily fabricated. It would also be desirable to provide a circuit which provides a signal path between a selected one of multiple signal sources and a common signal path while providing highly isolated signal paths between the unselected signal sources and the common signal path.